N. Wu, O. Wasynczuk, Purdue University; E. A. Walters, C. E. Lucas, PC Krause and Associates, Inc; Peter T. Lamm, U.S. Air Force Research Laboratory
Power systems that include regulated power-electronic sources and/or loads are susceptible to potentially destabilizing interactions between these components. A variety of techniques and methodologies have been developed to characterize the small- and large displacement stability of such systems. Perhaps the most common approach is to establish the input/output impedance-versus-frequency characteristics of all sources and loads, whereby Nyquist- and/or Bode-inspired criteria may be used to characterize interconnected system stability. Essential to this methodology is a means of accurately and efficiently determining the input and/or output impedance-versus-frequency characteristics of the power electronic components that comprise the overall system. These frequency-domain characteristics can be established by (1) direct measurement, (2) exercising detailed simulations, or, more commonly, (3) using state-space average-value models. The primary disadvantage of using direct measurements is that the hardware must be available a-priori which makes it difficult and/or expensive to change or tailor the impedance characteristics if instabilities occur. Calculation of the impedance characteristics from detailed simulations is generally time consuming, especially if the low-frequency characteristics are needed, and little insight is gained as to how the impedance characteristics are affected by the various design parameters. Average-value models overcome the previous disadvantages; however, they introduce a new one. In particular, the derivation of an average value models is typically time consuming, especially if the circuit topology is complex and/or the power converter exhibits multiple load-dependent modes of operation. In this paper, an automated approach of establishing average-value models of power electronic converters of arbitrary complexity is set forth. The user-supplied inputs consist of a standard Spice-like circuit description (branch parameters and network graph) whereupon the input/output impedance-versus frequency characteristics are automatically and rapidly established. In addition to eliminating the need for the analytical derivation of average-value models, this technique readily permits the inclusion of secondary effects such as conduction losses, switching losses, and magnetic nonlinearities, to name a few. This technique has been successfully applied to characterize the output impedance of a one-quadrant dc/dc buck converter and a three-phase generator/rectifier source.
Proceedings 3rd International Energy Conversion Engineering Conference, August 15-18, 2005, San Francisco, CA.